Interlocking in large electricity-supply substations¿a fundamental approach

The paper briefly reviews the interlocking of electrical plant and switchgear in large transmission substations, and shows that grouping occurs in two categories according to the switching function; i.e. whether for access and maintenance or operational purposes. There is a brief discussion of interlocking requirements and of conventional interlock principles using both mechanical-key and electrical-bolt arrangements. The philosophy of interlocking is considered against a background of system faults which have occurred, and the inadequacies of incomplete interlocking are discussed. The bulk of the paper is devoted to the development of a new approach to interlock circuit design, which is based on fundamental principles and uses established network theorems, and which is shown to be easily adaptable to any primary switching arrangement, however complicated. It is shown that, contrary to previous experience, when using the new principles, the interlocking of a substation need not place constraints upon the primary circuit design, such as has occurred in the past with conventional schemes. It is also shown that former unorthodox switching sequences which required the defeating of interlocks can be performed with the interlocks in full operation. Two schemes of interlocking, one using d.c. supply sources and one using a.c. sources are developed, and a practical scheme for a 4-switch-mesh substation, with mesh-opening isolators, is described. It is suggested that the new scheme makes possible the comprehensive interlocking of a substation as required for fully automatic switchgear operation, and that it has application in remote automatic-instructor mimic diagrams at control centres.